Many wireless communications systems utilize spread spectrum communications techniques in an attempt to help increase the interference immunity, thereby improving overall performance. Spread spectrum communications techniques use more data bandwidth than necessary to increase a probability of successful data transmission while potentially reducing the impact of the data transmission on other communications systems operating in the vicinity. For example, in a frequency hopping spread spectrum wireless communications system, transmissions from a sender to a receiver occur over a series of frequency bands, wherein at any given time, the transmission occupies a subset of the frequency bands available to transmit data in the wireless communications system. Both the sender and the receiver know which frequency bands are being used so that the receiver may listen in on the frequency bands used to carry the transmission.
Since the transmission may take place over a number of frequency bands, it may be unlikely that an interferer can damage the transmission in its entirety. Furthermore, if some other communications system is operating in the vicinity, the changing frequency bands used in the transmission may minimize the impact of the transmission on the other communications system should the transmission interfere with the operation of the other communications system.
Frequency synthesizers in the sender and the receiver may be used to generate reference frequencies at about the center of each of the frequency bands, permitting the transmitting and the receiving of the transmission. However, in fast hopping communications systems, the frequency synthesizers may be required to change frequencies very quickly, sometimes on the order of nano-seconds. This may exceed the capabilities of many forms of frequency synthesizers.
One technique used to overcome the fast frequency switching requirement is to use multiple frequency synthesizers, with each frequency synthesizer configured to produce a reference frequency at about the center of each frequency band. For example, phase-locked loops (PLL) may be used to produce the reference frequencies required. Another technique utilizes a fixed frequency mode synthesizer in combination with a single-side-band (SSB) mixer.
One disadvantage of the prior art is that phase-locked loops may be physically large, therefore, a large number of PLLs may consume a disproportionate amount of space. This may lead to a larger than desired electronic device.
Yet another disadvantage of the prior art is that PLLs may consume a large amount of current. This may be especially true when multiple PLLs are in simultaneous operation, which may be necessary to meet the fast hopping requirements. The additional power consumption of the PLLs may require a larger power supply (a battery in a wireless electronic device) or a shorter device runtime or both.
A disadvantage of the prior art is that the fixed frequency mode synthesizer in combination with an SSB mixer may have high current consumption, may require a large number of signal traces, and may cause a lot of unwanted output spurs.